1. Field of the Invention
The present invention relates to an image forming apparatus for copying machines, printers facsimile machines and the like having a developing device that uses a two-component developing material containing both a toner and a carrier.
2. Description of the Related Arts
FIGS. 1 and 2 show a developing device 1 used in conventional copying machines.
The developing device 1 has a developing roller 3 provided in the developing portion 2 opposite the copying machine type photosensitive member 100. The developing roller 3 comprises an internal magnet roller 4 and external sleeve 5, wherein said magnet roller 4 is stationary in a non-rotatable state and said sleeve 5 is rotatable in the direction indicated by arrow a via a motor 6.
Developing material transport paths 9 and 10 are respectively provided at the front and back of the circulation transport portion 7 disposed at the back of the developing portion 2 and are further separated by a partition wall 8. One end of the back transport path 10 is extended to form the toner supply portion 11 (left side in FIG. 2). The transport paths 9 and 10 are connected via openings 12 and 13 formed in the partition wall 8 provided opposite both ends of the front transport path 9. Further, the transport paths 9 and 10 are respectively provided with spirals 14 and 15, which are rotatable in the directions of the arrows b and c, respectively, via the actuation of the motor 16. The back transport path 10 is also provided with a toner density sensor 17, the detection data output from which are transmitted to the microcomputer MC.
A toner supply path 22 is provided at the bottom of the toner hopper 21 in the toner supply device 20. A bottom opening 23 provided at one end of the toner supply path 22, so as to be positioned above the supply path 11 of the aforesaid back transport path 10. A supply spiral 24 is further disposed in the toner supply path 22, so as to be rotatable via the actuation of the toner supply motor 25.
The previously described developing device 1 accommodates in the front and back transport paths 9 and 10 a two-component magnetic developing material containing toner and carrier. The developing material is circulated in the counterclockwise direction in FIG. 2 in transport paths 9 and 10 via the rotation of the front and back spirals 14 and 15. That is, in FIG. 2, the developing material in the front transport path 9 is transported from the right side to the left side, so as to enter the back transport path 10 via a communicating path 13. On the other hand, the developing material in the back transport path 10 is transported from the left side to the right side, so as to enter the front transport path 9 via a communicating path 12. Furthermore, the developing material transported through the front transport path 9 is maintained on the exterior surface of the developing roller 4, so as to be delivered opposite the photosensitive member via the rotation of the sleeve 5, and thereby develop via the delivered toner the electrostatic latent image formed on the surface of the photosensitive member 100. The developing material that has expended toner in the region opposite the photosensitive member is again returned to the front transport path 9 via the rotation of the aforesaid sleeve 5. Accordingly, the toner density during the period the developing material is transported from the right side to the left side of the transport path 9 in the drawing is reduced, i.e., the ratio by weight of the toner to the carrier is reduced.
The reduced toner density is compensated by controlling toner supply as described below. The toner density sensor 17 detects the toner density of the developing material passing the region P3 (hereinafter referred to as "detection position P3") opposite said sensor 17 at predetermined intervals, and the detection data is transmitted from the sensor 17 to the microcomputer MC. In the microcomputer MC, the measured toner density is compared to a standard reference toner density for the purpose of controlling toner density. If the toner density is reduced below a predetermined reference density, the toner supply motor 25 is actuated with a specific timing so as to precisely supply toner in the developing material determined to have a low toner density by the aforesaid toner density sensor 17, and a predetermined quantity of toner is supplied through the supply path 11 via the rotation of the supply spiral 24. The supplied toner is transported from the left side to the right side in FIG. 2 via the rotation of the back spiral 15, so as to supply the toner in the aforesaid developing material determined to have a low toner density at a converging position P0 wherein the supplied toner joins the developing material.
The previously described developing device 1, however, has a disadvantage inasmuch as suitable toner density cannot be assured even when toner is supplied at the converging position P0 in an amount which corresponds to the amount of toner density reduction, because toner is consumed during the: process of transporting the developing material through the front transporting path 9 after the developing material toner density has been detected at the detection position P3.
When the previously described developing device 2 was used to continuously form images and the image densities of the produced images were measured, wide variations were confirmed between the image density (ID) and output voltage of the tone density detecting sensor 17, as shown in FIGS. 3 and 4. The solid line, broken line and chain line in FIG. 3 respectively correspond to the regions at the right side, center and left side of the developing device shown in FIG. 2.
That is, the developing device having circulating developing material as previously described provides toner supply control that is incapable of producing adequate density stability only by simply supplying toner in the portion of the developing material having reduced toner density.